There is an ongoing need to modify pressure-sensitive adhesives to meet the needs of new applications. In general, when additives are incorporated into pressure-sensitive adhesives to modify their properties, care must be taken to avoid a loss in peel adhesion or shear strength. This has prevented a wide use of thermoplastic materials as modifiers.
Major classes of pressure-sensitive adhesives include tackified natural rubbers; synthetic rubbers such as butyl rubber; and tackified linear, radial, star, and branched and tapered styrene block copolymers, such as styrene-butadiene, styrene-ethylene/butylene and styrene-isoprene; polyurethanes; polyvinyl ethers; acrylics, especially those having long chain alkyl groups; poly-.alpha.-olefins; and silicones.
Generally, when additives are used to alter properties of pressure-sensitive adhesives, the additives need to be miscible with the pressure-sensitive adhesive or to form homogeneous blends at the molecular level. Some types of pressure-sensitive adhesives have been modified with tackified thermoplastic elastomers, thermoplastics, and elastomers. For example, thermoplastic materials have been added to polymerized hot melt acrylic pressure-sensitive adhesives wherein the thermoplastic is a packaging material or recyclable tape backings. In these cases, the type and amount of thermoplastic material is controlled so that the thermoplastic material can function as a packaging material while avoiding degradation of the adhesive properties of the pressure-sensitive adhesive.
However, more often than not when a non-tacky thermoplastic additive is blended with a pressure-sensitive adhesive, reduction of the overall adhesive properties of the blend (as compared to the pressure-sensitive adhesive only) are observed. Thermoplastic polymers have been added to styrene block copolymer adhesives to reduce the tack of the resulting pressure-sensitive adhesives for application of protective sheets to large area surfaces.
Pressure-sensitive adhesives, whether modified or not have been used for more than half a century for a variety of purposes. Generally, pressure-sensitive adhesives are used in tapes wherein a tape comprises a backing, or substrate, and a pressure-sensitive adhesive. Typically, a pressure-sensitive adhesive adheres with no more than applied finger pressure and can be permanently tacky.
In the medical field, pressure-sensitive adhesive tapes are used for many different applications in the hospital and health areas. For most applications, tapes are applied directly to a patient's skin. It is important that the pressure-sensitive adhesive tape be compliant and non-irritating to the skin, as well as adhering to the skin without causing damage to the skin when the tape or adhesive coated article is removed. A particularly useful medical application for pressure-sensitive adhesive tapes and articles is in the field of transdermal patches. Such patches can be used as drug transport membranes or to attach drug transport membranes to skin.
Although pressure-sensitive adhesive tapes and articles are widely used in the medical field, pressure-sensitive adhesive tapes and articles find widespread use in many other applications. For example, transfer tapes can be used to adhere two surfaces together such as the flaps of packing material or fabric to a surface. However, transfer tape adhesives generally have little tensile strength and one solution has been to add glass fibers to provide tensile strength.
Another use is in the field of labels, which require a large variety of pressure-sensitive adhesives due to a wide variety of surfaces. However, the pressure-sensitive adhesives must be able to be cut easily without stringing out or oozing to permit efficient manufacturing processes.
Another use for pressure sensitive adhesives is as a means to adhere preformed pavement marking materials used as traffic control markings. In such applications a pressure sensitive adhesive is used to adhere wear resistant, flexible base sheets of preformed pavement marking materials into place to demark pedestrian crosswalks, stop bars, and lane delinators. Such pressure sensitive adhesives must withstand the shear forces associated with tire traffic, while maintaining adherance to the road over a wide array of temperatures.
Pressure-sensitive adhesives require a delicate balance of viscous and elastic properties that result in a four-fold balance of adhesion, cohesion, stretchiness and elasticity. Pressure-sensitive adhesives generally comprise elastomers that are either inherently tacky, or elastomers or thermoplastic elastomers that are tackified with the addition of tackifying resins.